Mammalian sarcoma viruses are recombinants between helper leukemia viruses and endogenous sarcoma viral sequences. The current project is directed toward understanding the organization of the virus genome and delineating the physical recombination map by two dimensional fingerprints of the RNase T1 resistant oligoribonucleotides of genome RNAs. In another aspect, the endogenous RNA tumor viruses are an important factor in determining cancer in many mammalian species. The cellular control mechanisms of expression of the viral sequences have been studied in the in vitro systems transcribing the viral genes. BIBLIOGRAPHIC REFERENCES: Young, H.A., Shih, T.Y. Scolnick, E.M. and Parks, W.P.: Steroid induction of mouse mammary tumor virus: Effect upon synthesis and degradation of viral RNA. J. Virol. 21: 139-146, 1977. Shih, T.Y., Young, H.A., Parks, W.P. and Scolnick, E.M.: In vitro transcription of Moloney leukemia virus genes in infected cell nuclei ad chromatin: Elongation of chromatin associated RNA by E. coli RNA polymerase. Biochemistry, in press, 1977.